Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the temperature durability of the engine components must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-based superalloys, and through the development of oxidation-resistant overlay coatings deposited directly on the surface of the superalloy substrate to form a protective oxide scale during high temperature exposure. Nonetheless, superalloys protected by overlay coatings often do not retain adequate mechanical properties for components located in certain sections of a gas turbine engine, such as the combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coating (TBC) systems formed on the exposed surfaces of high temperature components have found wide use.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. TBC systems capable of satisfying the above requirements generally require a metallic bond coat deposited on the component surface, followed by an adherent thermal barrier ceramic layer that serves to thermally insulate the component. Various ceramic materials have been employed as the thermal barrier layer, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), scandia (Sc2O3), or another oxide.
The bond coat is typically formed from an oxidation-resistant aluminum-containing alloy to promote adhesion of the ceramic layer to the component and inhibit oxidation of the underlying superalloy. Examples of prior art bond coats include overlay coatings such as MCrAlY (where M is iron, cobalt and/or nickel), and diffusion coatings such as diffusion aluminide or platinum aluminide, which are oxidation-resistant aluminum-base intermetallics. The bond coat is typically disposed on the substrate by a thermal spray processes, such as vacuum plasma spray (VPS) (also know as low pressure plasma spraying (LPPS)), air plasma spray (APS), and high velocity oxy-fuel (HVOF) spray processes.
Conventional bond coats are typically applied as a bi-layer construction wherein a fine powder is first deposited on the substrate to form a dense, low oxide layer. Commercially available HVOF systems are typically used to deposit this layer. It is generally recognized that conventional HVOF processes are sensitive to particle size distributions, generally requiring finer particles ranging from −45+10 μm. The fine particle layer serves to protect the substrate from oxidation and corrosion, but the low surface roughness of the layer results in inadequate adhesion of the ceramic material layer.
A coarse powder layer is then deposited over the fine powder layer to achieve a desired degree of surface roughness for adequate adhesion of the ceramic material. APS bond coating techniques are often favored for the coarse powder layer due to lower equipment cost and ease of application and masking. Adhesion of the ceramic material layer to an APS bond coat is promoted by forming the bond coat to have a surface roughness of about 200 microinches (about 5 μm) to about 500 microinches (about 13 μm) Ra (Arithmetic Average Roughness (Ra) as determined from ANSI/ASME Standard B461-1985).
Although APS-applied bond coats provide better TBC adhesion due to their roughness, the coarse powder layer is generally unsuitable as a protective coating system. The coarse powder layer is relatively porous and prone to oxidation damage.
Thus, conventional bond coats are applied as a bi-layer in separate processes with separate equipment configurations to achieve the desired characteristics of a dense, low-oxide protective layer, and the surface roughness of a coarse powder layer. This practice, however, requires maintaining both powders in inventory, as well as the different coating systems. The process is time consuming in that it involves set up for two different processes, and can result in rework of coated pieces due to equipment or powder mix-ups.
Accordingly, the art would benefit from an improved commercially viable process for applying a single layer bond coat from a single powder composition, with the bond coat having the desired properties of conventional bi-layer bond coats.